It has a few interesting features. As you can see in the pics, the leads to the board are clipped and not soldered. The other ones in the Amparchives are all soldered. Even one made the same year by the same people. Wonder why that is?

I think the amp has the original power tubes, branded Teonex. The reason why they could have lasted is because it is impossible to bias the tubes properly. I tried modern Tung Sols and maxed out the pot at 50% bias. I noticed that the amp has 82k grid leak resistors which is strange as I understood these amps we're only sold by mail order in England? I tired some 6550's and they bias correctly. I then piggy backed a 100k resistor to the 47k bias resistor with alligator clips and got the EL34's in range. Left the 6550's in for now.

Another reason might be that the amp seems hard wired for 250V and we get 230V from the wall. B+ is very low, 350-something. I don't really need more head room (have a super bass already), so I'm fine with that if it isn't a practical/electronic problem?

There was someone on the Marshallforum a few years back who had the exact same complaint with a '76 lead & bass 50. Low voltage and cold bias. Were they built that way on purpose?

I wouldn't like to swap the grid leak resistor to 220's as I've read that this increases bass. The amp is bassy enough as it is (shared cathode and 0.1 output coupling caps). It is a real beast when wound up though, aggressive but not too bright and piercing. Bass channel is good with pedals, but a bit muffled on its own. Not as useful as channel one on a normal bass amp, I'd say.

I'm thinking of taking it to a tech to swap out the bias resistor before the bias pot. What would be the right value as 47k piggy backed with a 100k seemed good, 33k?

Does everything seem in order in the amp otherwise, as far as the pics can tell?

Played the amp with EL34's today. Something isn't quite right as I'm getting oscillation and squealing with the volume above 5. Weird thing is, with my attenuator I can dime it without problems. Did not notice this with the 6550's. Sockets are a little wiggly.